For a while, when you purchased a HD film from the iTunes Store it automatically also downloaded the SD version. For people with small hard drives, this is a pretty massive waste of space, so Finer Things in Tech suggests you delete those old copies.

iTunes has stopped automatically downloading both the HD and SD version of films, but if you've got a bunch of old films on your hard drive it's probably worth looking to see if you can delete some doubles. Just head into your iTunes' movie folder, usually ~/Music/iTunes Media/ and check through each of your movie folders. If you have both, you'll have one labeled (HD) and one without a label. You can delete that one without a label.